High molecular porous films having large numbers of fine continuous pores are being used in various applications such as a separator membrane that is used in the production of ultrapure water, purification of drug liquids, water treatment, and the like; a waterproof, moisture-permeable film that is used in the clothing, sanitary materials, and the like; and a battery separator used in batteries or the like. As a method for producing such a porous film, there is known a method of melt mixing polypropylene with inorganic filler such as calcium carbonate or barium sulfate, forming a film from the mixture, and then subjecting the film to stretching and porosification. However, the porous film obtainable by this method gives rise to areas of dispersion failure due to the low compatibility of inorganic fillers with polyolefin, and thus has a problem that pinholes are likely to occur during the process of stretching and porosification, or that the inorganic filler comes off during the production or during use, contaminating the process. As another production method, there has been suggested a method of heat dissolving liquid paraffin or the like in ultrahigh molecular weight polyethylene or the like to obtain a gel-like sheet, subjecting the gel-like sheet to a solvent removal treatment before and after stretching, and thereby obtaining a porous film. However, since the production method for the film includes solvent removal steps, there are problems that the production process is complicated, and that the high temperature heat resistance is insufficient because a polyethylene resin having a low melting point is used as a main component.
Furthermore, various methods for stretching a polypropylene sheet containing β crystals have also been suggested. For example, Japanese Patent No. 3443934 (Patent Document 1) suggests a porous film which is obtainable by stretching a β crystal-containing polypropylene sheet containing an amide compound. Furthermore, PCT International Patent Application WO 2002/066233 (Patent Document 2) describes a method for producing a porous film made of polypropylene by sequentially biaxially stretching a polypropylene sheet containing needle-like β crystals.
In regard to the method for producing a porous film using β crystal polypropylene, which is represented by the publications described above, a method of increasing the longitudinal stretch ratio (hereinafter, referred to as “MD stretch ratio”) and the transverse stretch ratio (hereinafter, referred to as “TD stretch ratio”) has been generally employed for the purpose of increasing pores in the film and thereby enhancing the air permeability properties. Furthermore, from the viewpoint of placing emphasis on the isotropy of the resulting film, it has been considered as common knowledge for those having ordinary skill in the art, that it is preferable to set the stretch ratios in the two directions to be almost equal. That is, in regard to the formation of a porous film using β crystal polypropylene, it has been considered preferable to stretch the film isotropically in the longitudinal direction and in the transverse direction at a high ratio. A film obtained by this method has satisfactory permeability characteristics because of the high porosity, and acquires a tensile strength which is isotropic in the longitudinal direction and the transverse direction of the film.
Furthermore, with regard to a porous polypropylene film using β crystals, an investigation has been conducted to improve handleability by enhancing the film surface sliding property by means of the surface unevenness generated at the time of stretching, as disclosed in Japanese Patent No. 3508515 (Patent Document 3).    Patent Document 1: Japanese Patent No. 3443934    Patent Document 2: WO 2002/066233    Patent Document 3: Japanese Patent No. 3508515